The invention concerns generally the technology of coupling a radio telecommunication device to some external apparatus like an external antenna or a testing arrangement. Especially the invention concerns solutions with non-galvanic coupling to the radio telecommunication device. A portable radio telephone or a mobile telephone is used as an examplary radio telecommunication device, but the invention is equally applicable in connection with other kinds of radio telecommunication devices like pagers, portable data terminals, positioning devices and so on.
Arranging communication connections between a radio telecommunication device and the external world necessitates in the former an antenna for receiving and transmitting electromagnetic radiation at a predetermined frequency band. In mobile telephone systems the telephone communicates through its antenna with a base station located somewhere at a maximum distance of some tens of kilometres from the telephone. However, in some situations it is preferable to temporarily arrange the communication connections of the radio telephone differently. Typical such cases are the use of the radio telephone together with a vehicle kit including an external antenna, coupling the radio telephone to some other kind of booster kit for establishing a wider coverage area, or coupling the radio telephone to a testing arrangement for measuring its performance either at the production line or during service.
A widely accepted solution for the above-mentioned special occasions is to have a galvanically coupled pair of radio frequency connectors, of which one is located in the telephone and the other is at the end of a coaxial cable or similar coupling means for conducting the radio frequency signal to the external equipment. Building a radio frequency connector into a radio telephone reserves space from inside the device and increases manufacturing costs, which are serious drawbacks because the industry is continuously striving to make especially handportable mobile telephones smaller and cheaper. There is also a danger of the galvanic connectors becoming dirty, corroded and/or misaligned even in the course of normal use, which weakens the quality of the achieved connection.
The European published patent application number EP 0 399 975 discloses a solution for coupling a mobile telephone to the cable of an external antenna with no galvanic connections. The solution is based on a pair of capacitive plates, one of which is located inside the dielectric cover of a mobile telephone and the other in a holder for holding the mobile telephone during use inside a car. When the mobile telephone is in its holder, the capacitive plates come near each other in a parallel configuration and form a coupling capacitor for the radio frequency signal to pass through. A separate switch is used to couple either the internal antenna or the capacitive plate to the transmit and receive ports of the transceiver in the mobile telephone. The performance characteristics of this solution are not very good, and the switch takes also space from inside the mobile telephone and increases manufacturing costs.
From the British published patent application number GB 2 266 997 there is known a structure like the one in FIG. 1. The mobile telephone 101 comprises a protruding antenna 102 and a dielectric outer cover 103. A cable 104 to an external antenna (not shown) comprises at its one end a coupler 105 which can be fastened to the outer cover of the mobile telephone by means of a Velcro tape or other disconnectable fastening means. A resonator 106 is located inside the coupler 105. When the coupler is connected to the mobile telephone, the resonator 106 lies within the electromagnetic field of the antenna 102 and mediates an electromagnetic coupling between the antenna 102 and the external antenna cable 104. The drawback of this structure is the pronounced dependency of the coupling characteristics on the mutual position of the antenna 102 and the resonator 106. Even a small change in the fastening position of the coupler 105 will dramatically change the coupling coefficient that describes the relative amount of electromagnetic energy transferred between the antenna and the resonator. In practice it is very difficult to align the coupler perfectly each time in a series of repeated connecting and disconnecting.
The European patent application with serial number 96660046.2 discloses a structure like that in FIGS. 2a and 2b. Here again no superfluous connectors or switches are needed, because the radio frequency signal is coupled between the protruding antenna 201 and the nearby resonator element 202 by electromagnetic coupling. A tuning element 203 and a ground connection 204 serve to enhance the quality of the connection. The drawbacks of the solution of FIG. 2 are that it is somewhat cumbersome in use and it requires the mobile telephone to have a protruding antenna.
The object of the present invention is to present an antenna coupler for coupling a radio telecommunication device to an external apparatus without the characteristic drawbacks of the prior art solutions. Especially it is an object of the invention to provide an antenna coupler that is easy to use and effective in the sense that a relatively small portion of the electromagnetic energy involved is wasted into unwantedly diffused radiation. A further object of the invention is to provide an antenna coupler with which it is easy to achieve an essentially identical connection of good quality even in repeated occasions of coupling and decoupling.
The objects of the invention are achieved by providing, for radio telecommunication devices with planar integral antennae, an antenna coupler with at least one antenna element which is similar to an antenna element in the radio telecommunication device. To further comply with the objects of the invention, the antenna coupler may be embedded into a holder for the radio telecommunication device.
The antenna coupler according to the invention is characterised in that it comprises
a second planar conductive antenna element which is essentially similar to a first planar conductive antenna element in the radio telecommunication device,
a first conductive ground plane parallel to said second planar conductive antenna element and
transmission means for conducting a radio frequency signal between said second planar conductive antenna element and an external device.
The invention applies equally to an arrangement for coupling a radio telecommunication device to an external apparatus. The arrangement according to the invention is characterised in that it comprises in an antenna coupler the abovementioned features.
A planar integral antenna is a relatively novel antenna structure for radio telecommunication devices, especially handportable mobile telephones. It comprises an essentially planar conductive antenna element with carefully designed outline and dimensions. There are one or several feedpoints for coupling the antenna element to the antenna port of a radio transceiver, and a ground plane somewhere in the close vicinity of the antenna element. According to the invention an essentially similar antenna element is provided in an antenna coupler. Additionally the antenna coupler comprises a ground plane. The antenna element in the antenna coupled is further coupled to a coaxial cable or other signal conducting means for conducting a signal in the transmission direction to some external device and/or a signal in the reception direction from some external device. Such an external device may be for example an external antenna belonging to a vehicle kit or a testing arrangement for measuring the performance of the radio telecommunication device during manufacture or service.
The dimensioning and mutual placement of the antenna element and the ground plane in the antenna coupler can be used in a way described in more detail below to tune the performance characteristics of the antenna coupler into some required direction. In a variation of the invention the radio telecommunication device comprises several differently sized planar integral antenna elements, in which case it is possible to use an antenna coupler with an equal number of equally sized planar antenna elements. The invention is not limiting in this aspect: for a radio telecommunication device with only one antenna element it is possible to use an antenna coupler with several antenna elements, and vice versa.